Overhaul xxboot. And merge floppy boot, taken from boot_ustar.
- Rewrite boot.S completely.
boot.S now supports boot from SCSI HD/CD and floppy.
- Use IOCS call to identify the floppy format, instead of chkfmt.s which
accesses hardware directly.
- Import print_hex() debug function from boot_ufs.
- Import a feature that displays initial registers (for debug) from boot_ufs,
and restore it (this in boot_ufs has been broken).
- Add size optimized alternatives for some libkern routines.
- Stop linking libsa to prevent to link unexpected objects.
- Bump version to 2.0.

Reduce binary size as possible. These reduce filesize about 20 words.
- Use word alignment (rather than longword alignment) for text/data section.
- Use short reference. In the bootloader, all symbols can be expressed in
short.

- Restore display of first_kbyte. This is helpful for development.
- Check the filesize of resulting file instead of section sizes.
If there is a gap between sections, the filesize may be exceeded even
though the section total is not exceeded. Of course that doesn't
usually happen but I've experienced this during development.

Revert placing .bss right after .data, and make .bss fixed at 0x5000.
.bss placed right after .data was overwritten when first 1KB loads full
.text+.data.

Check whether the text+data+bss doesn't reach 0x6000 where secondary
bootloader should be.

Make .bss follows .data to detect size restriction easier.
Previously, if text+data exceeds the restricted size, it will conflict
with manually fixed .bss area and ldscript makes it an error.
By this change, ASSERT() can detect it.

Use ASSERT() in ldscript to check first_kbyte (etc) restrictions,
rather than complicated shell expressions in Makefile.

Remove commented out ALIGN() operation.
It looks unnecessary and has never been used.

Pass address constants from Makefile to ldscript.

branches: 1.1.2; 1.1.6; 1.1.44; 1.1.50;
Initial support for xxboot (cdboot_cd9660, specifically).